System for navigating aircraft



April 13, 1948. H. M. LEwls 2,439,663

SYSTEM FOR NAVIGATING AIRCRAFT Filed 26, 1942 4 Sheets-Sheet l AprilA 13, 1948. H. M. Lawn; 2,439,663

SYSTEM FOR NAVIGATING vAIRCRAFT Filed Feb. 26, 1942 4 Sheets-Sheet 2 Time- H K enbaJ Kauenba: fw d d,

L INVENTOR l HAROLD MLEwls A RNEY April 13, 1948- v H. M. I Ewls 2,439,663

SYSTEM FOR NAVIGATING AIRCRAFT FIGB:

INVENTOR HAROLD M. LEWIS RNEY April 13, 1948. H. M. LEWIS SYSTEM FOR NAVIGATING AIRCRAFT.

Filed Feb. 26, 1942 4 Sheets-Sheet 4 n4 m Y RE E N 0L R TM. o D mmf, A Rf Vl B Patented Apr.. 13, 1948 SYSTEM FOR NAVIGATING AIRCRAFT Harold M. Lewis, Allenhurst, N. J., assignor, by mesne assignments, to Hazeltine Research, Inc., Chicago, Ill.,l a corporation of Illinois Application February 26, 1942, Serial No. 432,430

4 Claims.

The present invention relates to systems for navigatingan aircraft and, particularly, to such systems of the radiated carrier-wave type. While the invention is of general application. it is particularly suitable forfnavigating aircraft along a predetermined course to effect a landing oper-ation, for example, upon the surface of the ground or water or perhaps the deck of an aircraft car-'- rier, and will be described in connection with a system of this nature for effecting a landing on the ground.

It isfrequently necessary that aircraft be own blind; that is, during times when the pilot is unable to see the ground as when flying at night or under adverse weather conditions when the ground is obscured by elouds'or fog. It is sometimes desirable during such times that an aircraft, having ilown to its destination, be landed blind. This occurs, for example, when the pilot of en airplane having flown to his destination finds that the ground'is obscured by fog, that the ceiling is substantially zero, and that other near by airports to which the aircraft might safely be flown are also shrouded in fog. It becomes necessary at such times that the pilot attempt a landing even though the ground does not become visible until the airplane is within a few feet of it.

While'numerous proposals have been made for effecting blind landings by aircraft, only one of these has been adopted to any extent by commercial airlines. According to this method, the aircraft is guided along a radio beam to its desired destination and, when near its destination and while traveling the beam at a relatively high altitude, it passes over a marker station, thereby to inform the pilot that he is at a certain point on the initial. step of apredetermined landing procedure 1for the airport which he is approaching. Subsequent steps of. this landing procedure may require that his glide continue along the radio beam for a certain interval of time, that he execute a prescribed angle offturn which is ef- 'fected by banking the airplane at a certain angle `for a certain interval of time, and that contact with the ground be established at a time when the pilot is yet ying at an altitude of several hundred feet. Theoretically, thisl method of blind landing is effective even though sight contact with the ground is not established by the pilot until he is within a few feet of the ground, but in practice the possible errors in the instruments upon which the pilot must rely in effecting a landing in this manner renders the system impractical and dangerous where the ceiling is substantially zero, that is, when dense fog covers the airport upon which the pilot is required to land.

It would be desirable to provide a navigational system by which an aircraft can consistently and accurately be directed along a prescribed landing course from a. relatively high altitude to a point within a few feet of the ground. Such a system would insure a safe landing vof aircraft even in total darkness or when the ground is completely obscured by dense fog. It would further be desirable that a system of this nature employ a rather simple. compact, and inexpensive apparatus, yet one having a high degree of reliability. A system of this type should lend itself readily to adaptation to some form of automatic control by which the plane is automatically navigated along a desired course, or at least should require no more than a single indicating device which it is necessary for the pilot to observe where he himself navigates the craft in effecting a blind landing.

It is an object oi the invention, therefore, to provide a .new and improved system for navigating an, aircraft and one which avoids one or more of the disadvantages and limitations of prior art systems of this nature.

It is a further object of the invention to provide a new and improved system for navigating an aircraft in which a very large number of possible navigational courses for the aircraft are established by the system, any desired one of which may be selected at will by the person operating the aircraft.

It is an additional object of the invention to.

possesses the desirable features previously referred to.

In accordance with thev invention, a system for navigating an aircraft along 'a prescribed course toward a carrier-wave reflecting surface to effect .a landing operation thereon comprises a pair of 3 carrier-wave radiating systemseach supported in spaced relation above a carrier-wave reflecting surface, and means for applying a frequencymodulated carrier wave to each of the radiating systems for radiation therefrom. The system includes means carried by the aircraft and adapted to receive the carrier waves radiated by the radiating systems and responsive to the difference frequency of carrier-wave energy traveling directly and by reflection from the carrier-Wave reecting surface from each of the systems to the receiving means for deriving for each of the radiating systems a control signal. Each of the radiating systems has the characteristic that its` equal-valued control-signal loci are represented by geometric surfaces of revolution. The system alsoincludes means for simultaneously utilizing a preselected value of each. of the control signalsk to aid in the navigation of the aircraft along the aforementioned prescribed course to effect the landing operation.

For a better understanding of the present invention, 'together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawings, and its scope will be pointed out in the appended claims.

Referring now to the drawings, Fig. 1 is a schematic circuit diagram of a complete system for navigating aircraft embodying the present invention; Figs. 2 and 3 are graphs used as an aid in explaining the operation of the invention; Fig. 4 is a schematic circuit diagram of a complete system for navigating an aircraft embodying the invention in a modified form; and Figs. 5 and 6 are graphs used in explaining the operation of the Fig. 4 arrangement.

Referring now more particularly to Fig. l. of

the drawings, there is represented a complete system for navigating aircraft, for example an airplane P, embodying the present invention in a particular form. The system includes a carrierwave radiating system having a substantially nondirectional radiation characteristic in a horizontal plane. Where the navigation system is used to direct aircraft to effect a landing on the ground, the radiating system preferably comprises a vertical dipole antenna I0, I'I. The radiating system I0, II is relatively :fXedly supported in spaced relation from the ground G, which exhibits the properties of a carrier-wave refiecting surface.

The radiating system supporting means comprises a tower I2 itself supported on a platform I3 adapted by means of wheels I4 to be readily transported from one position to another, as desired. While the tower as a whole may be movable, it is relatively fixed as far as the aircraft is concerned. The use of a mobile radiating systemis of greater importance in a modified form of the invention presently to be described. The navigation system also includes means for applying to the radiating system, for radiation therefrom, a carrier wave a characteristic of which is periodically varied. This means comprises a source of modulation signals or modulation-signal generator I5, which is coupled to a modulation circuit of a frequency-modulation oscillator I6, and is operated by a synchronous motor I1,

which is energized from an alternating-current source I8. The output circuit of the unit I6 is coupled to the input circuit of a radio-frequency amplifier I9 of one or more stages, the output circuit'of the latter unit being coupled to the dipole antenna I0, 'I I. v

The navigational system also includes means adapted to receive the carrier wave radiated by the radiating system and responsive to carrierwave energy traveling directly and by reflection from the ground to the airplane P for deriving a control signal.' The receiving means is carried by` the airplane P and comprises an antenna 20, 2| coupled to the input circuit of a detector or modulator 22, the output circuit of which is coupled to an amplifier 23 of one or more stages. There is coupled to the output circuit of the amplifier 23, in the order named, a limiter system 24, a frequency detector 25, and an amplifier 26. The receiving means also includes means for utilizing the control signal derived by the receiving means to aid in the navigation of the aircraft along a desired course, this means comprising an indicating device 21, for example a voltmeter, which is coupled to the output circuit of the amplifier 26.

Considering now the operation of the system described, and referring to the curves of Fig. 2, a modulation signal, preferably of symmetrical saw-tooth wave form, is generated by unit I5 and applied to a modulation circuit of unit I6 to frequency-modulate the carrier vwave generated by the latter unit periodically over a predetermined range of frequency deviation, as represented by the solid-line curve A. This frequencymodulated carrier wave is amplified by the amplifier I9 and applied to the dipole antenna I0, II for radiation therefrom to the airplane P.

Assume now that the airplane P is flying at a relatively low altitude and at a predetermined distance from the radiating system I0, Il. A small time interval t1 will elapse before the radiated carrier wave in traveling over the direct path between the radiation system I0, II and th'e airplane P is received by the latter, the frequencydeviation characteristic of the received wave being as represented bythe solid-line curve B. The radiated carrier wave also travels over an indirect path to the airplane P by reflection from the ground G and, since this indirect path is longer than the direct path between the radiating system I0, Il and the airplane P, a longer time interval t2 elapses before the reflected carrier wave is received by the airplane P, the reflected carrier wave having a frequency-deviation characteristic as represented by the broken-line curve C,

There is thus applied to the input circuit of the detector or modulator 22 two carrier waves which differ in frequency during the interval t3 by a constant difference frequency FA. The detector or modulator 22 combines or modulates these carrier waves with each other to derive in its output circuit a beat-frequency control signal which has the frequency-deviation characteristic represented by curve a. It will be evident that the maximum frequency of the control signal Varies directly with the difference between the time interval t1 and the time interval t2, the former varying with the distance of the -airplane P from the radiating system I0, II and the latter varying both with the distance of the airplane P from the radiating system and also with the altitude of the airplane above the ground. This control signal is selected and amplified by the amplifier 23, is limited to a substantially constant amplitude by the limiting system 24 and applied to the frequency detector 25. The frequency detector 25 derives a unidirectional control potential varying in magnitude with the maximum frequency of the control signal. 'I'his potential is amplified by the amplifier 26 and applied to the indicating device 21 which is preferably calibrated aaaaeea s in units of maximum lfrequency of the control signal. e

If it be assumed that the airplane P is at the same distance from the radiating system l0, il

'and the frequency-deviation characteristic of th'e control signal derived by"the detector 22 is as reprented by the curve b. Similarly, if the airplane P is at the same distance from the radiating system il),4 il as in the ilrst two examples, 'assumed but at a yet higher altitude, the carrier wave received by the airplane P by reflection will now have a frequency-deviation characteristic as -indicated by the broken-line curve C" and the frequency-deviationcharacteristic of the control v signal derived by the detector 22 will be as represented by the curve e. f

The manner in which the maximum frequency image-radiating system Ill', ii' and the airplane P for several maximum-frequency valuesV of the control signal derived by th'e detector 22,'are represented in Fig. 3 by the ,curves J, K, L. M and N. It will now be evident that even though the airplane P is at a given distance from the radiating system l0, Il, the maximum frequency deviation oi.' the control signal derived by the where: I

Fd=the maximum frequency deviation of the con-- deviation of the control signal derived by the detector 22 varies with' the distance and altitude of the airplane P with respect to-the radiating system Ill, li will be more clearly evident from the curves of Fig. 3. It may be noted here that the reflection ofj v the radiated carrier wave does not occur at the actual level of the ground G but actually occurs along-an eective ground plane G' located somewhere between a few inches and several feet below the surface of th'e ground, de-

pending upon the nature of the'soil and the frequency or the radiated carrier. wave, but in general parallel therewith. Since the angle of incidence is equal to the angle of reection ofthe reflected carrier wave :at the effective ground plane, the latter may be considered for convenience as being radiated from an image-radiating system lil', li located directly beneath the radiating system i6, il and at thes'ame distance below the effective ground plane G' as the radiating system l0, Ail is above this plane. IIt will now be evident that the maximum'irequency 'of the control signal derived by the detector 22 is proportional to the dierence of the space displacements -ofthe airplane P from the radiating system i, i i

and the image-radiating system. it', il', the latter space displacement being the same, of course, as the length of the path actually traveled by the radiated carrier signal from the radiating system lil, il to the eilective. ground-radiation plane G' and to the airplane P. Since the curve generated by a point moving so that the4 dinerence of its distances .from two xed points is always constant is a hyperbole., it will be evident thatl the radiating system has the characteristic that equal-valued control-signal frequency loci are represented approximately by hyperbolic surfaces of revolution, each symmetrically disposed about. the radiating system and concave upward f rom the ground. Equal-valued control-signal loci of this general nature are also characteristic of the system disclosed in applicant's copending application, Serial No. 407,732, filed August 2l,

1941, entitled "System for-'determining the position of an object in space," now Patent No. 2,a06,953, issued September 3, 1946. Traces of equal-valued control-signal loci in a vertical plane, including the radiating system l0, li, the

detector 22 -nevertheless varies with the altitude of the airplane P since the space displacement of the airplane P from the image-radiating system it', it varies with. thealtitude of the airplane P above the eiective ground planea'.

It can be shown mathematically that:

trol signal derived by the detectar 22, Fdvr=the range of frequency deviation of the carrier signal radiated by the radiating system lull, Tm=onehalf the period ci the modulation signal derived by unit i5, and N=the number of cycles of the radiated carrier wave during the time interval ts, which, of course, varies with the diner-ence between vthe time intervals t1 and t2, Fig. 2, and thus with the differences in the path lengths of the carrier wave in traveling directly and by reflection to the airplane P.

' From Equation-1 it will be seen that all of the parameters of the navigational system are estab.- lished by the selection of a range of frequency deviation of the radiated carrier wave and by the selection of the period of the saw-tooth modulationsignal derived by the unit i5. Once these parameters have been established, the maximum frequency of the control signaliderived by the detector 22 .varies only with the parameter N which, as 'pointed out, varies with the difference between the space displacements of the airplane P fromthe radiating system iii, l i and the imageradiating system it', Il. Consequently. for any given navigational system embodying the invemv tion and having preestablished values for the xed parameters of the system, it willbe known that each of the equal-valued control-signal loci characteristics of the radiating system lil, I i pass between the latter and ground at a predetermined altitude. For example, it will be known that the equal-valued control-signal loci for a, signal of say 100 cycles maximum frequency becomes tangent to the ground directly beneath the radiating systemi0, il, for example as represented by curve K, Fig. 3.

In using the arrangement of the invention, an airplane in -approaching the radiating system ill, i i will pass through regions of equal-valued control-signal locil of increasing frequencyand will continue towards the radiating system ill, l i at some constant arbitrary altitude until the maximum frequency of the control signal derived by the detector 22 corresponds to that which it has previously been' established is the desired course for the airplane to follow from any high altitude to a lower altitude, generally within a few feet of the ground, necessary to effect a landing at a desired point on the ground; The indicating intrument 21 will tell the pilot at that time th-at the desired navigational course has been reached and the pilot will thereupon so navigate the airplane that a. constant indication is thereafter provided by the indicating device 2l. Assuming that such course is that represented sight of the tower I2, since he must then be in,-

sight of the ground and, .of course, cannot continue on the course without impacting the tower.

I2. It will thus be evident that'the pilot may safely effect a blind landing even though the ground is shrouded in dense fog, it being only necessary in such case that the tower I2 be suitably illuminated or otherwise made visible to the Ipilot at a sufllciently early moment to avoid collision.

Since any time delay required fo'r the radiated carrier wave to travel from the radiating system I0, II to the airplane P results in an effective phase shift of the frequency-modulation component of' the carrier wave with respect to the same component of the modulation signal generated by unit I5, it will be evident that the detector 22 necessarily comprises means adapted i to receive the carrier wave radiated by the radiating system and responsive to the relative phases of two modulation signals derived from individual carrier waves, one of which travels lover a direct path and the other of which travels over an indirect path to the receiving means, for derivlng a control signal.

It has been stated that in using the arrangement of Fig. 1, the pilot must use care in flying along .the desired navigational course that he does not fly into the tower IZ. The need for the pilot to watch out for and avoid striking the tower supporting the radiating system is avoided in the modified form of the invention of Fig. 4 which is essentially similar to that of Fig. 1, similar circuit elements being designated by similar reference numerals and analogous circuit elements by similar reference numerals with alphabetical subscripts. from that of Fig. 1 in that a pair of carrier-wave radiating systems and carrier-wave translating channels is provided on the ground and a pair of carrier-wavereceivers is carried by they airplane. Thus, one of the radiating systems comprises a dipole antenna Ill. Il and the other radiating system comprises a dipole antenna Illa, Ila, the first-named radiating system having coupled thereto a signal-translating channel comprising the frequency-modulation oscillator I6 and the amplifier I9, While the second radiating system has coupled theretoa signal-translating channel comprising a frequency-modulation oscillator |63 and an amplifier I9a'. The modulation-signal generator I5 is coupled to a modulation circuit of each of the frequency-modulation oscillators 1I 6 and Ia. The carrier waves generated by units I6 and' I6.; are simultaneously deviated over different but adjacent ranges of frequency Adeviation by the modulation signal generated by unit "I'he airplane carries a first carrier-wave rev ceiver comprising units 22--26 coupled to'an antenna 20, 2| 'and adapted to receive the carrier wave radiated by the first radiating system I0, II, and also carries a second carrier-Wave receiver comprising units 22a- 26a coupled to a second antenna 20a, 2Ia and adapted to receive the carrier wave radiated Iby the second radiating system Illa, Ila. -The output circuit of the amplifier 26 of the first carrier-wave receiver includes a load resistor 28, while the output circuit of the The system of Fig. 4 diiil'ers amplifier 28s of the second carrier-wave receiver similarly includes a load resistor 29, the resistors 28 and 28 having a common Junction Il). An indicating device 21', which may be a zero-center scale type of voltmeter, is coupled across the uncommon terminals of the -load resistors 28 and 29. 'Ihe radiating systems III, II and I'Ih. I I arel supported in spaced relation to eachother and at equal heights above the ground.

Ihe operation of this-modified form of the vin vcntion is essentiallysimilar to that of the Fig. 1 arrangement' except that the carrier waves radiated by the radiating systems I0, Il and III., IIs are simultaneously modulated by the modulation signal generated by unit Il, but have different mean frequencies in order that each radiated carrier wave may be received by an individual one of the carrier-wave receivers 22-48. inclusive, and 22a-26., inclusive. It will be understood .that the carrier wave received by each of the carrier-wave receivers travels to the receiver from the radiation systeml over both a direct path and an indirect path by reflection from the ground, whereby the detector 22 derives,A

one control signal the maximum frequency of which varies with the difference of the length of the direct. and indirect paths traveled by the' there is derived therefrom a control potential which after amplification by the amplifier 26 appears across the load resistor 28. Likewise, there is derived from the second control signal 'by the frequency detector 25. a second control potential which after amplification by the amplifier 26s appears across the load resistor 29.

difference between the magnitudes of the con- 25 and 25a which, in turn, varies with the frequency of the respective control signals derived by the respective detectors 22 and 22a. It will be evident that when the airplane P is flying a course equidistant from the radiating systems I0, il and I 0s, IIs, the control signals derived fby the detectors 22 and 22.. have the same maximum frequency and the indicating device 21' is' not deflected from its zero-center scale position.

That this is true will be more evident from the curves of Fig. 5 wherein the equal-valued control-signal 'loci vcharacteristics ofthe radiating system I il, II are represented by the solid-line curves J to N, inclusive, and the corresponding equal-valued 'control-signal loci characteristics of the radiating system |09., Ila are represented by the broken-line curves Jn to Nn, inclusive. I.An airplane 'flying parallel to the ground at any altitude and equi-distant from the radiating systems Ill, II and IDD., Ils will be flying in the plane represented by the broken line RR and it will be evident that the maximum frequencies of the control signals derived by the detectors 22 and 228. are equal at any altitude of the airplane P when flying at any given distance from the radiating system.

In using this modified form of navigating sysy tem a pilot will fly towards the navigating system Consequently, the indicating device 21 measures'the aesaees 9 tant between the radiating systems i0. I I and in, il using the indications of indicating device 21 for thiI purpose. He will continue on this course until e indications provided by the indicating device 21`iniorm him that .he has reached the preestablished navigational course of thesystem which he is thereafter to follow in effecting a landing at a given point on the ground. In this arrangement. as in the arrangement of Fig. l, the equal-valued control-signal loci characteristie ci' each of the radiating systems i0, Ii and i, Ils are represented by geometric surfaces of revolution symmetrical about each radiation system and concave upward from theground,

there being the diil'erence in the present arrange-v ment that the intersection of a predetermined v loci of one of the radiating systems with a predetermined loci of the other defines the desired navigational course for the airplane in effecting a landing. Thus, referring to the curves of Fig. 6, the solid-line curve S is an isometric view of the intersection of the' loci represented by the curves J and Je of Fig. 5 and is assumed to be the desired navigational course for the airplane P. The Pilot follows the course ,S by maintaining a predetermined constant indication of the indicating device 21 and by maintaining zeroscale deflection of the indicating device 2l' and in doing so is caused to navigate his airplane from 'a relatively high altitude to within a few feet of the ground at a point equidistant between the towers i2, |25 which support the respective radiating systems l0, I-I and Ille, Ila.- Since the course S if followed past a point located directly between the towers i2, lis would cause' the airplane to gain altitude, 'thepilot is aware when his plane levels off that he is midway between the towers i2, H2B and further knows that he is within a few feet of the ground and thus is enabled at this timeto put his plane into the stall required for the final landing operation. In this way he is enabled to effect a completely blind landing even though neither the ground nor any object is at any time visi-ble to him.y

The towers t2, iin are mobile and preferably are located toward one edge of the landing field and on the side opposite that from which the prevailing wind is blowing in order that'the pilot may land into the wind in conventional manner. While the indicating devices 2ll and 2l' of the carrier-wave reflecting surface, means for applying a frequency-modulated carrier wave to each of said radiating systems for radiation therefrom, means carried by said aircraft and adapted to receive the carrier waves radiated by said radiating systems and responsive to the difference frequency of carrier-wave energy traveling directly and by reflection 'from said carrier-wave reflecting surface from each of said systems to said receiving means for-deriving for each of said 'radiating systems a control signal, each of said radiating systems having the characteristic that its equal-valued control-signal loci are represented by geometric surfacesof revolution. and means for simultaneously utilizing a preselected value of each of said control signals to aid in the navigation of said aircraftv along `fsaidprescribed course to effect said landing operation.

` 2. A system for navigating an aircraft along a prescribed course toward a carrier-wave reiiectin'g surface to effect a landing operation thereon comprising, a pair of carrier-wave radiating systems each supported in spaced relation above a carrier-wave reflecting surface, means for applying a frequency-modulated carrier wave to each of said radiating systems for radiation therefrom, means carried by said aircraft and adapted to receive the carrier waves radiated by said radiating Asystems and responsive to the dierence I ing surface, and means for. simultaneously uti- Fig. a iarrangement ar'e shown as individual indieating devices, it will be evident that these devicesl may be built as a unitaryindicating device. In the latter event; the indicating devices 2l and 271' maintain their identity but the needles of each are preferably supported one behind the other and arranged to pivot about a common axis. This is to be desired since the pilot need then only observe one instrument in effecting a blind landlng- While there have been described what are at present considered to be the preferred embodiments of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover allsuch changes and modifications as fall within the true spirit and scope of the invention.`

What is claimed is: l

1. A system for navigating an aircraft along a prescribed course toward a carrier-wave reflecting surface to e'ect a landing opration thereon comprising, apair of carrier-wave radiating systems each supported in spaced relation above a lizing a preselected value of ea'ch of said control signals to aid in the navigation of said aircraft along said prescribed course to effect said landing operation.

3. A system for navigating an aircraft along a prescribed course toward a carrier-wave reflectingv surface to enact a landing operation thereon comprising, .a pair of carrier-wave radiating systems each supported in spaced relation to a carrierwave reflecting surface, means for applying a fresaid radiating systems and responsive to the dif- 'ference frequency of carrier-wave energy traveling directly and by reflection from said carrierwave reecting surface from each of said systems to said aircraft for deriving for each of said radiating systems a control signal, each of said radiating systems having the characteristic that its equal-valued control-signal loci. are represented by geometric surfaces of revolution and the intersection of a predetermined loci of one'oi said radiating systems with a predetermined loci of the other of said radiating systems defining said prescribed navigational course for said aircraft, and means for simultaneously utilizing a preselected value of each of said control signals to aid in the navigation of said aircraft along said prescribed course to eiect said landing operation.

fi. A system for navigating an aircraft along. a prescribed course toward a carrier-wave reflectlng surface to eect a landing operation thereon y 11 comprising, a pair of carrier-wave radiating systems supported in spaced relation to each other and to a carrier-wave reflecting surface, said radiating systems having a, substantially nondirectional characteristic in a horizontal plane, means for applying a frequency-modulated carrier wave to each of said radiating systems for radiation therefrommeans carried by said aircraft and adapted to receive the carrier waves radiated bysaid radiating systems and responsive to the difference frequency of carrier-wave energy traveling directly and by ree'ction from said carrierwave reflecting surface from each .of said systems to said aircraft for deriving for each of said radiating systems a control signal, each of said radiating systems having the characteristic that its equal-valued control-signal loci are represented by geometric surfacesrof revolution symmetrically disposed about said each radiating system and xed relative thereto and the intersection of a predetermined loci of one of said radiating systems with a predetermined loci or the other of said radiating systems defining a suitable navigation'al course for directing said aircraft toward 12 said carrier-wave reflecting surface to effect a landing, and means for simultaneously utilizing a preselected value of each of said control signals to aid in the navigation of said aircraft along said course. A

HAROLD M. LEWIS.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS 

